Notes about the schematic:
MICROCONTROLLER:
Schematic specifies a 16F876, but this is probably overpowered for
most applicatoins. A less expensive 16F627 is adequate for most
applications.
COMMUNICATION
The PIC chip communicates via standard RS-232 serial ports. Note that for
interfacing to a computer serial port, a RS-232 line driver such as a
MAX233 is required. Via this serial port, the orb can communicate
with the pager module, a computer, palmtop, or any device with a serial
port.
LEDs
A very common mistake is to think of light emitting diodes (LEDs)
simply as luminous devices which conduct current in one direction.
While this is true, it does not take into account that all diodes (LEDs
included) have a characteristic "forward voltage drop". An
understanding of this property is important if you don't want to fry your
LEDs.
As a general rule, red LEDs have a forward voltage drop around 1.6
volts, and green and blue LEDs have a voltage drop of 3.4 volts.
Therefore, wiring 6 red LEDs in series gives a total voltage drop of 9.6
volts, while wiring two sets of three green / blue LEDs in series gives a
total voltage drop of 10.2 volts. These voltage drops are close
enough all LEDs can be driven from the same unregulated power supply
described above without any heat dissipating current limiting
resistors.
For more information on LEDs
http://www.maxim-ic.com/eequiz/eequizanswer21.htm
http://www.google.com/search?q=LED+forward+voltage+drop
http://ledmuseum.home.att.net/
POWER
The board is powered via a 9-volt 300ma unregulated transformer. Use of a higher voltage transformer will fry
the LEDs (unless the LED topology is changed). Note that an unregulated transformer only supplies the
rated voltage when under load. If no load is attached, a 9-volt
transformer will float around 12 volts. Whatever transformer you
choose, be sure to check the current flowing through the LEDs doesn't
exceed limits.
PULSE WIDTH MODULATION
Each bank of LEDs is controlled by a pair of PIC outputs. This is to compensate for the
highly non-linear relationship between brightness and duty cycle. For
example, the difference between duty cycles of 1% and 2% ON is very noticeable, while the difference between 74% and 75% is very slight.
One set of PWM outputs is connected directly to the LED bank and gives a
high-current coarse brightness control. The other set of PWM outputs
is connected through a current limiting resistor and gives a low-current
fine control. This is especially useful at the low range of
brightness where "staircasing" would otherwise be very noticeable.
The current limiting resistor for fine control is large enough the power
dissipation is negligable.
For even greater control, use 3 or more PWM outputs per LED
bank. Or for a more economical orb, use just one.
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